Cathode-ray tubes



March 14, 1961 K. c. BURN 2, 7 ,326

CATHODERAY TUBES Filed Aug. 1'7, 1959 Inventor Kennafh C0//e7f Bum ttorn e y States CATHGDE-RAY TUBES Kenneth Collett Burn, Rusholm, Manchester, England, assignor to Ferranti, Limited, Lancashire, England, a company of Great Britain This invention relates to cathode-ray tubes of the kind utilising electrostatic deflection of the electron beam and having first and second pairs of deflector plates in that order from the electron gun.

In cathode-ray tubes of the kind specified the first pair of deflector plates usually has the greater deflection sensitivity and therefore the signal to be observed is applied to this pair, the time base signal being applied to the second pair of plates. In many cases it is desirable for the user to be able to adjust the deflection sensitivity of this pair of plates to increase the sensitivity to observe weak signals. Furthermore it is sometimes necessary for the manufacturer to be able to adjust the sensitivity of the plates to match the tube with several other tubes.

It is an object of the present invention to provide a cathode-ray tube or" the kind specified in which it is possibie to adjust the sensitivity of the first pair of deflector plates.

According to the present invention a cathode-ray tube of the kind specified includes two metallic plates disposed between the first and second pairs of deflector plates normal to the direction of the electron beam in its undeflected condition, each of said plates containing an aperture through which the electron beam passes, and electrical connections to said two plates whereby in operation an adjustable potential diflerence may be applied between said two plates to form an electron lens capable of determining the sensitivity of the first pair of deflector plates.

One embodiment of the present invention will now be described by Way of example with reference to the accompanying drawing which shows a fragmentary perspective view, partly in section, of a cathode-ray tube in accordance with the invention.

Referring now to the drawing the cathode-ray tube includes an envelope of known form comprising a conical portion 1 with a screen (not shown) at the base of the cone and an elongated neck 2 at the apex. The electrode system is mounted in the neck of the envelope and includes the usual electron gun 3 for producing an electron beam and first and second pairs of deflector plates 4 and 5 respectively.

2,975,326 Patented Mar. 14, 1961 In order to control the deflection sensitivity of the pair of deflector plates 4 two metallic plates 6 and 7 are mounted between the first and second pairs of deflector plates 4 and 5 normal to the electron beam in its undeflected position. The plates 6 and 7 are provided with apertures 8 and 9 respectively through which the electron beam passes, the long edges of the apertures 8 and 9 lying parallel to the direction of deflection of the first pair 4 of deflector plates. The plates 6 and 7 are provided with electrical connections 10 and 11 respectively to the exterior of the envelope so that an adjustable potential diflerence may be applied between the two plates 6 and 7 thus forming an electron lens. The power and sign of this lens depend upon the magnitude and direction of the potential diflerence applied between the two plates 6 and 7 and by varying the potential difference the deflection sensitivity of the first pair 4 of deflector plates may be increased or decreased.

The two plates 6 and 7 are normally operated at approximately the mean deflector plate voltage, but by altering the voltage on one or both plates by a few hundred volts positive or negative a considerable change may be made in the deflection sensitivity of the first pair 4 of deflector plates with an electron beam of several kilovolts energy.

In controlling the deflection sensitivity of the first pair 4 of deflector plates in this way the deflection sensitivity of the second pair 5 of deflector plates is not affected. Furthermore, the two plates 6 and 7 serve to screen the first and second deflector plates-4 and 5 from each other so that the normal inter-plate shield may be omitted.

The aperture provided in each of the two plates 6 and '7 need not necessarily be formed as a rectangular slot as shown in the drawing. It may, for example, be a circular aperture.

What I claim is:

l. A cathode-ray tube of the kind having first and second pairs of electrostatic deflector plates in that order from the electron gun including two metallic plates disposed between said first and second pairs of deflector plates normal to the direction of the electron beam in its undeflected condition, each of said plates containing an aperture through which the electron beam passes, and electrical connections to said two plates for applying an adjustable potential difference between said two plates related to the mean deflector plate voltage to form an electron lens capable of varying the effective sensitivity of the first pair of deflector plates.

2. A cathode-ray tube as claimed in claim 1 in which said two plates each contain a rectangular aperture.

References Cited in the, file of this patent UNITED STATES PATENTS 

